(1) that when a current of electrified oxygen is passed through a tube containing dry zinc-sodium (which absorbs ozone, but has no action at all upon the so-called antozone) and then passed through water, no mist or vapour is produced, thus demonstrating that ozone and antozone are not simultaneously formed; (2) that when a stream of electrified oxygen is conducted through a tube containing chloride of calcium (which absorbs antozone but does not affect ozone), and then passed through water, clouds still appear in the receiver, which could not be the case if their existence depended on the presence of antozone; (3) that all the tests of the so-called antozone are identical with those of peroxide of hydrogen. Even Meissner has recently qualified the assent he formerly gave to Schönbein's hypothesis, which, although it has still a few supporters, is daily losing ground; and it may now be considered as at least provisionally established that the antozone of Schönbein, the atmizone of Meissner, and the peroxide of hydrogen of Thenard are identical. Babo, indeed, is of opinion that the cloud-forming power depends on the presence of nitrogen, but this appears to have been sufficiently refuted by Meissner himself.

Dismissing Schönbein's antozone from our consideration, it is now necessary that we should inquire into the volumetric relations of ozone and oxygen, the modes of preparing the former body, the circumstances which modify its production, and its distinctive properties; and also endeavour to substitute some valid theory of its constitution, since Schönbein's ingenious hypothesis seems no longer tenable.

Ozone may be prepared in at least seven different ways: (1.) By passing electric sparks through air or oxygen, or by the inductive influence of a series of sparks passed along the outer surface of the tube containing the gas. This latter is the preferable modification of the method we are now considering, because the passage of the actual spark destroys a large portion of the ozone as soon as it is formed. Sparks an inch long generate twice as much ozone as those of one sixth of an inch. It is a curious

fact that the form and covering of the containing tube exert an immense influence on the amount of ozone produced by this method. Thus, the production of ozone in a row of parallel uncovered tubes is nil; when these are coated and fastened together with a thin covering of wax the quantity produced is appreciable; it is increased when lateral glass wings are affixed to the tube, still further augmented when sealing-wax supports are added to the glass wings, and reaches its maximum when the tubes are provided with thick glass wings with sharp angular edges. When the angles of these wings are rounded off the production of ozone falls to nil, but it is again formed in abundance on the angles being restored. These facts, however strange, seem to be perfectly well established by Meissner's experiments.

(2.) Ozone may be prepared by the electrolysis of acidulated water, when it appears at the negative pole. (3.) By placing a piece of phosphorus half covered with tepid water in a vessel of air. The phosphorus ought to be removed after two hours at latest, as it then begins to absorb a portion of the ozone it had previously formed.

(4.) By the action of strong sulphuric acid upon permanganate of potash, a method judiciously recommended. by Dr. Fox when the formation of this body is desired for the purification of the atmosphere in hospitals and theatres.

(5.) By dispersing water in a pulverized form through the air, as the electricity generated by the vaporisation of the water dust converts a portion of the atmospheric oxygen into ozone.

(6.) By the introduction of a heated glass rod into a mixture of air and ether-vapour.

(7.) By exposing almost any ether or essential oil to the action of light and air. Dr. Day, of Geelong, recommends that ethers thus ozonized should be employed to disinfect the clothes, bedding, bandages, &c., of the sick in hospitals. A convenient method of producing ozone in hospital wards, &c., is to heat a platinum wire to incandescence by means of a Bunsen's coil.

The theory of the constitution of ozone, as also that of

the volumetric relations subsisting between that body and oxygen, seem now to be established on a tolerably secure basis by the labours of Andrews and Tait, supplemented by those of Sorel. The first-named physicists observed that while only a small proportion of the oxygen experimented with could be converted into ozone at one time, a certain reduction of the volume of the gas attends the formation of each successive portion of this substance. By the application of heat the ozone was reconverted into oxygen, and the total volume of the gas resumed its first dimensions. They found that 100 volumes of oxygen, when acted on by the electric spark, contracted to 92 volumes, with the formation of ozone; and, strange to say, when the ozone thus formed was absorbed by mercury, 92 volumes of oxygen still remained. This process was continued, the residual 92 volumes were ozonized and thereby reduced to 84.82 volumes, the ozone thus formed again taken up by mercury, and still 84.82 volumes of oxygen remained behind. This latter phenomenon seemed inexplicable, but Dr. Odling accounted for the formation of the gas by the supposition that the molecule of ozone contains three atoms of oxygen, and, since the molecules of all gases are equal in volume, was, therefore, half as dense again as the molecule of oxygen, which contains only two atoms. Hence, when the 100 volumes of oxygen contracted to 92 volumes with formation of ozone, what took place was this:-1 molecule, 16 volumes, or 2 atoms of oxygen united with half a molecule, 8 volumes, or 1 atom of oxygen to form 1 molecule, two atoms, or 16 volumes of ozone, thereby diminishing the total bulk of the gas by 8 volumes, i. e. redueing the 100 volumes to 92 volumes. A similar process occurred with each successive formation of ozone. Odling further suggested that when the gas is acted upon by mercury it is only the third atom of the oxygen contained in the molecule of ozone which is absorbed by the metal, the remaining two atoms being liberated as free oxygen, and the total volume of the gas, accordingly, remaining unchanged. This theory was confirmed by Soret, who, by using oil of turpentine as the absorbent

(which takes up the whole molecule of ozone), found that only 76 volumes were left of the 92 volumes of ozonized oxygen-a result which exactly fulfils the necessary conditions of Odling's hypothesis. Odling's doctrine may, therefore, be considered as tolerably well established, even although in 1872 MM. E. and P. Thenard were disposed to impugn it, on account of the results of some experiments they had instituted upon ozone with sulphate of indigo and arsenious acid.

Did space permit, I think it would not be very difficult to show that these results may be explained in a way perfectly reconcilable with Odling's hypothesis, while the explanation offered by Thenard himself seems both indefinite and hardly sufficient to account for the phenomena he observed.

The presence of ozone in the atmosphere, which is still doubted by Frankland, was called in question by Dumas, Thenard, and others, more especially by M. Cloez, who showed that the reaction with starch and iodide of potassium can be produced by chlorine, bromine, nitrogen, and acetic and formic acids, alone or in combination. This led Bérigny, in 1865, to propose the question of the existence of atmospheric ozone to the French Academy, who appointed a committee to investigate this interesting subject. In the name of this committee M. Frémy denied the presence of ozone in the atmosphere, as he considered the oxidation of metallic silver to be its sole conclusive test, and this reaction he was unable to obtain. He subsequently endeavoured to show that whilst ozone must constantly be formed in the atmosphere by thunderstorms, its existence could be but momentary; since the same electrical disturbances must produce nitric acid, which would destroy the ozone as soon as produced.

Houzeau had satisfactorily demonstrated the uncertainty of the iodide of potassium and starch test several years before, and had substituted in its stead the alkalinization of a weak solution of iodide of potassium, which he conclusively proved could be effected only by ozone or peroxide of hydrogen. The further investigations on this subject which he was led to make by the publications of Cloez in

1861, and by the subsequent denial of the existence of atmospheric ozone by Frémy on the part of the French Academy in 1865, only served to confirm his previous conclusions, while he demonstrated the unsuitableness of the metallic silver test by showing that this substance can only be oxidized by ozone when that body is present in comparatively large quantities. He further showed that ozone might really exist constantly in the air notwithstanding its acknowledged instability, since the generation of it is incessant, and its stability much augmented (as is the case with many other bodies) by its state of dilution.

Absolute certainty as to the existence of atmospheric ozone was not, however, obtained even thus, since peroxide of hydrogen (as well as ozone) possesses the power of alkalinizing a solution of iodide of potassium. This source of ambiguity was to some extent removed by the protoxide of thallium test proposed by Schönbein, although, as the presence of carbonic acid interferes with this reaction, it is more applicable to laboratory than to meteorological purposes. This, together with the researches of Andrews, Huizinga, and Gorup-Besanez, has now established beyond reasonable doubt the presence of ozone in the atmosphere.

As Schönbein proved that antozone, "atmizone," or peroxide of hydrogen (which we have seen to be identical) is formed to a greater or less extent simultaneously with ozone in most of the reactions by which the latter is prepared; as it is also produced by the slow combustion of various organic and inorganic substances; and as in Russia it has been demonstrated to exist in snow-water, its presence also in the atmosphere may now be deemed established. be distinguished from ozone by the fact that it only has the power of alkalinizing a solution of iodide of potassium in the presence of carbonic acid, but its most distinctive test is its power of further oxidizing the protoxide of lead.

It may

The more or less frequent occurrence of ozone in the atmosphere having been thus established, we must next inquire at what times it is present, and what circumstances determine its scarcity or abundance.